1. Field of the Invention
This invention relates to data output device and also to a data output method. More particularly, it relates to a data output device to be suitably used for an AV server in a broadcasting station.
2. Description of the Prior Art
The increased number of channels that have become available for accessing to various types of information due to, if partly, the development of CATV (cable television) in recent years has by turn given rise to a strong demand for novel audio/video data recording/reproducing apparatus adapted to recording and reproducing different set of audio/video data concurrently with a single apparatus. As a matter of fact, apparatus referred to as a video server (or an AV(audio and/or video) server) and adapted to concurrently recording different sets of audio/video data in and also reproducing such data from randomly accessible recording means such as hard disks have getting popularity to meet this demand.
Generally, a video server installed in a broadcasting station is required to show a high data transfer rate and have a huge recording capacity in order to meet various requirements including a high image and sound quality and a long recording time. Thus, attempts have been made to produce a high data transfer rate and a high recording capacity by introducing a data recording/reproducing apparatus capable of dealing with a plurality of hard disk (hereinafter referred to as HD) units concurrently and storing parity data in the apparatus in advance so that the overall reliability of the apparatus may be maintained if one or more than one of the HD units fail. With such an arrangement, it is possible to realize a multi-channel video server adapted to a variety of applications such VOD (video on demand) and NVOD (near video on demand) systems, where same source data comprising a plurality of different sets of audio/video data are recorded in a distributed fashion and then reproduced simultaneously or with slight time differences before they are transmitted through a multiple of channels.
The data recording/reproducing apparatus used in such a video server is based on the RAID (redundant arrays of inexpensive disks) technology of utilizing a plurality of hard disk drives (hereinafter referred to as HDDs), each being adapted to drive a plurality of hard disks, as described in Patterson et al. ("A Case for Redundant Arrays of Inexpensive Disks (RAID)", ACM SIGMOND Conference, Chicago, Ill., Jun. 1-3, 1998.)
According to the above paper, the RAID technology may be classified into five categories, RAID-1 through RAID-5. Category RAID-1 used to write same data in a pair of HDDs. With Category RAID-3, the input data are divided into groups of data having a predetermined length and recorded in a plurality of HDDs, while a parity data is generated as exclusive OR of the corresponding data blocks of each HDD and written in another HDD. With Category RAID-5, on the other hand, data are divided into large units (blocks) and the data of each unit are recorded in an HDD as data block, while the out come of exclusive OR (a parity data) of the corresponding data blocks of each HDD is recorded in other HDDs as a parity block in a distributed fashion.
For the remaining aspects of RAID, reference should be made to the above identified paper.
An AV server as described above comprises a data storage consisting of a plurality of hard disks for storing AV data, a plurality of AV data input/output interfaces (hereinafter referred to as ports) for inputting AV data into and outputting such data from the data storage and a system controller for controlling the operation of the entire system. Each port has a memory for regulating the timing of retrieving our AV data from and that of storing AV into the data storage. Then, an AV server having the above described configuration performs processing operations concurrently for a multiple of channels as the system controller controls the ports in such a way that they are used evenly and cyclically on a time division basis with a predetermined period. More specifically, time slots are allocated to the ports for processing AV data for a channel so that each port performs an input/output processing operation within the time slot allocated to it to realize multi-channel input/output processing operations. Therefore, with such an AV server, time slots are allocated to the ports of the multiple of channels periodically in a rigid fashion. Then, as a general practice, the length of each time slot is made as short as possible in order to minimize the queuing time of each port, provided that the time length required for each port to read and reproduce a given volume of data from the memory at an ordinary rate is secured.
An AV server used by professionals in broadcasting stations and other similar facilities are required by necessity to operate accurately on a frame basis. Thus, with such an AV server, the delay from the time when a data reproduction command is received to the time when the requested AV data are actually reproduced is often required to be fixed (hereinafter referred to as fixed delay). On the other hand, for each of the channels of the AV server to reproduce data continuously without any intermission, said processor should simultaneously control the operation of reading out the specified data from the data storage and storing them in a memory and that of outputting a data already stored in the memory.
With the known AV server of the type under consideration, it is a general practice that the memory of each channel comprises three or more than three banks because a reproduction command is transmitted regardless of the timing at which each channel can access the data storage. Differently stated, each memory is required to have at least three banks in order to secure a fixed delay time regardless of the timing at which the reproduction command is received and said processor is required to carry out its control operations such that any discrepancy of timing may be appropriately. dissipated.
However, the three-bank arrangement of each memory can be disadvantageous in terms of cost and there has been a demand for a two-bank arrangement for memories.